The present invention relates to an apparatus and a method of aligning a vehicle's axles and especially to a method and apparatus for aligning a fixed axle relative to the frame and suspension elements of the vehicle.
A proper aligned front end of a vehicle has long been a necessity for long tire life, handling ease and for achieving optimal fuel economy. The vehicle wheels need to travel in a straight line, with all wheels pointing in the same direction. That is, all four wheels must be square to each other and square to the road surface. All four wheels need to be parallel to one another and perpendicular to a common center line with the wheels straight up and down.
Full attention has generally focused on the alignment of the front wheels. However, a misaligned rear axle also results in reduced tire life and reduced fuel economy. A misaligned rear axle causes excessive tire drag and creates undesirable lateral forces. Even in new trucks, studies have shown that most are in need of rear axle alignment to align the rear axle relative to the suspension element and rear tires relative to each other and frame of the vehicle. The vehicle wheels need to be parallel to one another. Because of the increased tire wear and reduced fuel economy, even small misalignments of the rear axle become particularly important for freight carrying vehicles that must travel long distances. However, obtaining the proper alignment in an assembly line setting is difficult as are the minute adjustments that must be made quickly and accurately.
The present invention is directed towards the correction of fixed axle skew which is when the rear axle cocks slightly sideways causing both the rear tires of the vehicle to point slightly in the wrong direction or in a slightly different direction from the front wheels. In addition, if a fixed axle housing is slightly bent, one wheel will have skew relative to the other wheels which will also increase tire wear on the one wheel while reducing fuel economy in the vehicle.
The thrust line in a vehicle is an imaginary line created by the direction in which the rear wheels are pointing. The difference between an imaginary line drawn down the center of the vehicle and the thrust line equals the thrust angle. The thrust angle or axle offset causes both rear tires to point in the wrong direction and creates skew in the rear wheel assembly.
It has been common to perform a front end alignment using laser based wheel alignment systems. The axis about which the front wheels of an automobile or truck turn as it travels down the road must be carefully set to minimize tire wear, for safety and for stable handling characteristics. The orientation of these axes is determined by three angles. The toe-in angle, which specifies the angle between the rim of the wheels and a line drawn parallel to the direction in which the vehicle is pointed and the camber angle, which specifies the angle between the rim of the wheels and vertical and the caster angle, which specifies the angle between the vertical and the axis about which individual wheels turn when changing direction. These angles are specified individually for each wheel for each model and make of a vehicle. It must be periodically tested and reset to ensure continued economic and safe vehicle performance. However, in addition to front wheel alignment, it is desirable to set the thrust angle of the rear axle to avoid rear axle skew and also to ascertain that the rigid axle is not slightly bent to thereby cause skew in one of the rear wheels. In addition, the rear toe can also be adjusted.
The present invention is directed to correcting the thrust of a rear or any fixed axle to reduce small amounts of skew in the wheels and to thus prolong tire wear and improve fuel economy. A laser alignment system is used for aligning both the front wheels and the rear axle in which a laser is attached to each vehicle wheel spindle or hub, as set forth in the Loescher U.S. Pat. No. 6,823,598.